1. Field of the Invention
This invention relates to a motor servo circuit for subjecting a motor to feedback control so as to hold the rotational velocity of the motor constant, and for controlling the rotational phase of the motor when required.
2. Description of the Prior Art
A motor servo circuit has been known which exercises feedback control in such a manner that the rotational velocity of a motor will be held constant and includes a rotational velocity sensor for sensing the rotational velocity of the motor, a velocity error sensing circuit for sensing a rotational velocity error by comparing the sensed rotational speed with a given reference velocity, and a phase compensating circuit which includes a capacitor, for controlling a motor driver circuit based on the velocity error signal inputted thereto. In order to control rotational phase, the motor servo circuit was also provided with a rotational phase sensor for sensing the rotational phase of the motor, a rotational phase error sensing circuit for comparing the sensed rotational phase with a prescribed reference phase to sense the difference between the two, and arithmetic means for adding the sensed rotational phase difference and the rotational velocity error (or for subtracting one from the other) and applying the result to the phase compensating circuit.
In a conventional motor servo circuit of this type, the rotational velocity error became large in magnitude and a large signal was applied to the phase compensating circuit when the motor was started or when the reference velocity changed. As a result, the capacitor in the compensating circuit becomes charged and it was impossible for the servo circuit to exercise control until this DC charge is discharged. Consequently, considerable time was required for the motor to attain a steady velocity, namely a servo-locked state.
In order to solve this problem, there has been proposed a motor servo circuit [Japanese Patent Application Laid-Open (KOKAI) No. 61-129772] provided with a circuit for sensing whether the servo-locked state has been attained, with the capacitor being short-circuited by a diode when the locked state was not attained. With this circuit, the capacitor was discharged through the diode so that the time required to attain the servo-locked state is shortened.
However, since the capacitor is short-circuited by the diode, the open-loop gain of the servo system declines and certain problems result. Specifically, the circuit could not deal with large fluctuations in load, the motor failed to start when starting was attempted, and a very long period of time was required to attain a steady velocity.